The present invention relates to a wear-resistant titanium or titanium-alloy member and a method for manufacturing the same, and more particularly to a member, such as a poppet valve, rocker arm, piston pin, or the like of an internal combustion engine, which requires lightness in weight and good wear resistance, and a method for manufacturing such a member.
Poppet valves, rocker arms, piston pins, etc., used in internal combustion engines of automobiles and the like are expected to be reduced in weight, in order to improve the fuel economy and the responsivnness of the engines during acceleration or deceleration, and to lower the level of noise. Materials used for these members include lightweight, heat-resistant titanium alloys, such as a Ti-6Al-4V alloy. To maintain the desired engine life, the titanium-alloy members have tentatively been surface-hardened, for higher wear resistance, by plasma spraying of hardening build-up material, ion-nitriding, work-hardening, etc.
According to the plasma-spraying method, however, the poor cohesiveness between the hardening build-up layer and the base material may lead to a possible cracking and/or separation at the interface. The method of ion-nitriding, which is free from such cracking or separation of the build-up layer, cannot provide a hardened layer of a required thickness. By the work-hardening method, moreover, a satisfactory member life cannot be ensured.
Stated in Japanese Provisional Patent Publication No. 56-150183, for example, is a method in which the surface of a titanium-alloy member is melt-hardened by means of a DC TIG arc in a gas-mixture atmosphere containing nitrogen and an inert gas, such as argon. This method, as compared with the ion-nitriding method described above, can provide a thicker hardened layer. By this prior art method, however, some members still cannot be given a satisfactory hardened layer depth. When this method is applied to an engine poppet valve of a automobile engine, in order to harden its valve face to be seated on a valve seat, for example, an allowance for final machining must be secured to shape the surface of a melt-hardened layer. In consideration of such an allowance, the hardened layer depth provided by the conventional melt-hardening method is not thick enough. Therefore, if the melting state of the valve face is maintained for a long period of time to obtain a desired hardened layer thickness by this method, the so-called welding damage occurs, and hence leads to an improper member surface. If the melt-hardening operation on the valve face is repeated to obtain a desired hardened layer depth, moreover, the manufacturing cost will inevitably increase.